1. Field of Invention
This invention relates to electrical fuses and is particularly related to an electrical fuse having an improved fusible element which is useful for the protection of electrical appliances with low rated current capacity.
2. The Prior Art
Electrical fuses having low rated current capacity are well known and have been widely used for protection of electrical appliances. Fuses having low rated current capacity of the order of 100 m A or less usually require a fusible element, preferably made of silver wire having a diameter of the order of 10 .mu.m or less. However, silver wires with such extremely small diameter are difficult to fabricate and, in addition, they lack the requisite mechanical strength and structural integrity. As a practical matter, it is difficult to fabricate silver wires having a diameter smaller than about 45 .mu.m.
In order to overcome the aforementioned difficulties and permit the use of fusible elements made of silver wire with the desired small diameters, it has been suggested to use, as the fusible element, a monofilament yarn made of a plastic material such as polyacrylonitrile wherein the surface of the filament is either chemically coated or it is electroplated after chemical coating in order to make an electrically conductive filament for use as a fusible element. One of the drawbacks of these fusible elements is the relatively low melting point of the coating on the yarn surface which is necessarily limited by the softening point of the coated polyacrylonitrile filament yarn, i.e., 125.degree. C., or less.
Another type of fuse suggested in the prior art employs an insulated film of a high molecular plastic material as a supporting member, the surface of which is coated with a suitable metal to make the fusible element. The problem with this type of fusible element is that the high molecular weight plastic support member is heat sensitive and is readily deformed by the thermal expansion caused by excessive current flow. Moreover, excessive current flow through the metal coating causes it to crack, and therefore, current flow may be prematurely interrupted. Also, repeated rise and fall in temperature during the current flow adversely affects the physical properties of the plastic support material and could result in its permanent and irreversible deformation, with consequent instability of the fuse.
Accordingly, it is an object of the present invention to provide an improved fuse for use in electrical appliances having low rated current capacity.
It is another object of this invention to provide an improved fuse having a novel and unique fusible element which exhibits excellent performance and improved fusing characteristics when used in electrical appliances having low rated current capacities as low as about 1 milliampere and as high as about 250 milliamperes.
It is still another object of this invention to provide such fuses which are more stable and more durable than comparable prior art fuses.
The foregoing and other objects of this invention will become more apparent from the following detailed description thereof taken in conjunction with the accompanying drawings.